This invention relates to shift timing controls in automatic shifting power transmissions, and more particularly, to shift timing controls in such transmissions wherein a fluid operated friction device is responsive to a pressure time relationship during engagement and disengagement.
In prior art transmissions, an accumulator is utilized to provide for the proper pressure time interval during a speed change to provide the least disturbance during a shift transient. The accumulators have historically been utilized only to control the on-coming friction device, which is generally a disc type fluid operated structure. The off-going friction device is generally permitted to freely exhaust, and the accumulator control is usually removed from the exhausting circuit and exhausted either separately or through a separate restricted passage, such that the accumulator has no affect on the off-going device.
These systems would let the engine accelerate freely to the new speed requirement for the on-coming gearing ratio. If the ratio interchange or transition is made onto a freewheeler rather than a friction device, the shift timing will be correct and the shift quality will be fairly smooth. However, it would be more economical if the off-going friction member could be controlled or otherwise modulated at a desired disengagement rate during the ratio interchange. This is particularly important in what is known as a clutch-to-clutch type shift where time is required and some action has to be taken to apply the on-coming clutch at synchronization while releasing the off-going clutch.
To obtain satisfactory shift quality, it has been proposed that pulse width modulated solenoid valves be utilized to control both the on-coming and off-going clutch or friction devices. This requires sufficient electronic control and ECU capabilities to permit the independent control of the pressure at each friction device. While these devices do provide for good shift quality, they also increase the cost of the transmission control. It would be advantageous if a friction device could be accompanied by an accumulator which would function in both the on-coming and off-going states during ratio interchanges.